Resistance of the two-spotted spider mite, Tetranychusurticae (Koch), to acaricides previously effective against it on strawberries in California has reopened consideration of biological control. The authors considered the reproductive capacities of the prey and predator species, voraciousness of the predators, comparative motility of both predatory and prey species, and acceptance of alternate foods, if any, on the part of predatory species. Comparison of inherent powers of increase of predatory and prey species is not a proper criterion for determining whether the predator can control a prey species. Other things being equal, a prey-specific predator of lesser voraciousness (per individual) is more reliable in preventing outbreaks, Within limits, use of alternate foods may add to, rather than detract from, the effectiveness of a predator in the population control sense. Years of field and laboratory research and observations involving specific work of the authors and others in strawberries, grapes, citrus, deciduous fruits, and field crops support the view that phytoseiid mites, particularly members of the genus Metaseiulus, are often efficient predators of spider mites, and should be encouraged to a maximum consistent with profitable production.